1. Field of the Invention
The present invention relates to a method of forming a thin film of zinc oxide and a method of producing a semiconductor element substrate and a photovoltaic element by using the thin zinc oxide film, and more particularly to a technology of forming a thin zinc oxide film in a satisfactory state on only one surface of the substrate.
2. Related Background Art
In a photovoltaic element utilizing a semiconductor layer made of a material such as hydrogenated amorphous silicon, hydrogenated amorphous silicon germanium, hydrogenated amorphous silicon carbide, microcrystalline silicon or polycrystalline silicon, there has been utilized a reflective layer on the back surface (a surface of the semiconductor layer opposite to the light incident side) in order to improve the light collecting efficiency in the long wavelength region.
Such reflective layer desirably exhibits effective reflection characteristics in a wavelength region close to the end of the band gap of the semiconductor material where the absorption becomes lower, namely in a range from 800 nm to 1200 nm. Such condition can be sufficiently satisfied by a metal layer made of a material such as gold, silver, copper or aluminum.
Also in order to achieve so-called optical confinement, it is already known to provide a layer with surface irregularities which is optically transparent within a predetermined wavelength region. Such layer with surface irregularities is generally provided between the above-mentioned metal layer and the semiconductive active layer to effectively utilize the reflected light, thereby improving the short circuit current density Jsd. Also for preventing the deterioration of the characteristics by a shunt path, it is known to provide a conductive layer of a light-transmissive material, that is, a transparent conductive layer between the metal layer and the semiconductor layer. Broadly speaking, these layers are deposited by vacuum evaporation or sputtering and provide an improvement of 1 mA/cm.sup.2 or more in terms of the short circuit current density Jsd.
As examples of these configurations, the reflectivity and the textured structure of the reflection layer consisting of silver atoms are investigated in the Extended Abstract of the 51st Autumn Meeting of the Japan Society of Applied Physics (1990), 29p-MF-2, p.747 "Optical confinement effect in a-SiGe solar cell on stainless steel substrates" and Sannomiya et al., Technical Digest of the International PVSEC-5, Kyoto, Japan, p. 381, 1990, "P-IA-15 a-SiC/a-Si/a-SiGe multi-bandgap stacked solar cells with bandgap profiling". In these examples, effective irregularities are formed by depositing silver with a change in the substrate temperature to obtain a two-layered reflection layer, and an increase in the short circuit current is achieved by the optical confinement effect realized by the combination of the two-layered silver reflection layer with a zinc oxide layer provided thereon.
Such a transparent layer employed as the optical confinement layer is conventionally deposited by vacuum evaporation utilizing resistance heating, electron beam, sputtering, ion plating or chemical vapor deposition (CVD), but the drawbacks such as the high preparation cost of the target material or the like, the high depreciation cost of the vacuum apparatus and the low efficiency of material utilization result in an extremely high cost of the photovoltaic element utilizing these technologies, thereby constituting a major barrier in the industrial application of the solar cell.
As a countermeasure for such drawbacks, there has been reported the technology of forming a zinc oxide film by liquid phase deposition (Preprint of the 65th technical seminar of the Japanese Academy of Applied Physics (Fall 1995), p. 410 "Formation of ZnO film by electrolysis in aqueous solution").
This method which does not require an expensive vacuum apparatus or an expensive target can drastically reduce the manufacturing cost of the zinc oxide film. Also this method is effective for the photovoltaic element of a large area such as solar cells, because film deposition can be made also on a substrate with a large area. However, this electrochemical method of depositing zinc oxide has the following drawbacks:
(1) Since zinc oxide is deposited electrolytically on a conductive substrate, the zinc oxide film is deposited also on the back surface of the substrate, thereby causing deterioration of the work efficiency such as deterioration of the soldering property or of the adhesion property of an adhesive material on the back surface. Also when the substrate on which the zinc oxide film is formed by electrodeposition is supplied to the production process for a photovoltaic element, the solar cell characteristics may be deteriorated by degassing in the vacuum apparatus. Furthermore, transport failure may result in the post process of the roll-to-roll system due to fluctuation in the friction coefficient.
(2) A deposited film tends to generate abnormal growth of acicular, spherical or ramified shapes on the order of microns or more on the film, particularly when the current density or the concentration of the solution is elevated. When the zinc oxide film is employed as a part of the photovoltaic element, such abnormal growth may induce a shunt path in the photovoltaic element.